Petrogenesis of Gunbarrel magmatic rocks: Homogeneous continental tholeiites associated with extension and rifting of Neoproterozoic Laurentia

Abstract

The ca. 780Ma Gunbarrel Igneous Event of northwest Laurentia consists of spatially discrete suites of sills, dykes and lavas distributed over a vast area extending from Wyoming in the south to the Wopmay Orogen and the Mackenzie Mountains of Northwest Canada. Thick (≤100m) sills and rare dykes in Wopmay orogen and thinner (≤30m) sills, dykes and rare lavas in the Mackenzie Mountains are moderately evolved, augite+oligoclase–labradorite+ilmenite–magnetite gabbros and amygdaloidal basalts. Systematic petrochemical differences between units reveals that each is likely derived from subtly distinct parental magmas collectively exhibiting mutually consistent element variations. The dataset is remarkably homogenous, in particular, the incompatible trace elements and the Sm–Nd isotopes. All rocks preserve petrochemical evidence of an enriched MORB-like mantle source, but a small lithospheric component in the primary magmas resulted in elevated LILE, minor negative HFSE anomalies and sub-depleted mantle but supra-bulk earth ɛNd values. The lithospheric component was slightly older, modestly fractionated, Sr-depleted, garnet-free (pyroxenitic?) lower crust or, similar material that was previously recycled into the lithospheric mantle. Mineral chemical data for plagioclase and clinopyroxene in chill margin samples from a Hottah sheet in Wopmay orogen, indicates rapid and repeated turbulent mixing of geochemically and thermally similar magmas. These were staged from large, lowermost crust(?) magma chambers centred over an asthenospheric thermochemical anomaly thought to lie to the west of present-day North America. These magmas were then rapidly emplaced across western Laurentia.